1. Field of the Invention
This application relates to interference cancellation in a multiple antenna receiver, specifically as such interference cancellation applies to wireless telecommunications.
2. Description of the Related Art
When observing a cocktail party, a few characteristics relating to the sound are readily apparent. One is that individual conversations can be heard by the participants of those individual conversations and by those in close proximity thereto. The second is that to a person not taking part in a conversation and also not within close proximity to any individual conversation, the collective noise of the many conversations can be heard, but no individual conversation can be discerned from the general noise. In the first instance, the conversations can be heard because the signal (i.e., voice) to interference ratio is strong within close proximity to the speaker. In the second instance, the signal to interference ratio is low, and therefore interference from multiple conversations masks the individual conversations. Both observations, however, highlight the role that interference can have on sound quality.
The same phenomenon is applicable to wireless communications. Interference degrades performance, and unless the signal to interference (noise) ratio is sufficiently high, no particular signal can be discerned. In simplistic terms, that means the voice on the other end of a cellular telephone call may not be heard by a receiver of the call if the interference is too strong. Accordingly, cellular and other wireless communications systems have been designed to insure that the signal received by the wireless device is sufficiently strong relative to the interference such that the voice (and data) can be heard and understood.
Mobile telecommunication service providers and vendors are constantly striving to improve the quality and performance of cellular telephone communications. One such area of focus is the reduction of the interference caused from either (1) adjacent frequency carriers of a cellular base station, known by those skilled in the art as “adjacent channel interference,” or (2) adjacent cellular base stations operating at the same frequency carrier, known by those skilled in the art as “co-channel interference.” The problem of interference is exacerbated when the cellular system is operating within a tight frequency spectrum, especially and including GSM with a high traffic load.
As will be appreciated by those skilled in the art, similar to the so-called cocktail party problem, the interference observed by a cellular telephone multiple antenna receiver is a summation of individual interference from each of a variety of sources. Such sources of interference include, inter alia, adjacent channels within a base station and co-channel and adjacent channels from surrounding base stations, and may include interference from other sources as well. Because nothing is known about the individual signals causing the interference, it is difficult to extract the original voice signal; hence the cocktail party problem arises.
There are known techniques for reducing or canceling the noise in certain circumstances. For example, in analyzing the summation of multiple interference signals, it is possible to determine whether one particular interference source is dominant over the others contributing to the summation. A simple measure to capture this is the Dominant to the rest of Interference Ratio (DIR), which is the ratio of the power of the dominant interferer to the sum of the powers of the rest of the interferers plus No. This ratio is defined as:
  DIR  =            I      max                                ∑          k                ⁢                  I          k                    -              I        max            +              N        0            where Imax is the average power of the dominant interfering signal (co- or adjacent channel interference) and No is thermal noise of the system. In the case where there is a DIR much greater than one (1), there is typically a dominant interferer. In the case where the DIR is less than or approximately equal to one (1), then there is no dominant interferer.
Manufacturers and others in the telecommunications industry have been addressing the interference problem by designing interference cancellation features into their systems. One way to combat the interference is to use frequency hopping where the interference at a certain frequency will be distributed among multiple frequencies instead of being focused on one frequency. See, for example, “Interference Diversity Gain in Frequency Hopping GSM”, VTC 95, Hakan Olofsson, et al. Such interference averaging schemes are marginally helpful, but do not quite reduce or cancel the interference source to acceptable levels. Another example of interference mitigation is to use a technique known in the art as “Interference Rejection Combining” (IRC). See, for example, “A System Performance Evaluation of 2-Branch Interference Rejection Combining”, Stephen Craig et al. Each of the above-cited references is hereby incorporated by reference in its entirety. IRC is marginally effective where there is a dominant interference source, i.e., those systems where the DIR is much greater than one (1). For those situations where there is not a dominant interference source, the interference suppression techniques being developed by those skilled in the art are mostly ineffective and the benefits realized pale in comparison to the cost of implementation.
In various interference suppression techniques outside of the cellular telephone industry, there exist various methodologies relating generally to interference suppression. Using those known techniques, those skilled in the art will appreciate that if there is a series of N antennas forming a system, interference can be suppressed for all but one (N−1) of those antennas. This is because at least two actual signals must be observed for the known suppression algorithms to be applicable.
U.S. Patent Application of Meyer et al., Publication No. U.S. 2002/0141437 (the “Meyer Application”) addresses a method for interference suppression for TDMA and/or FDMA transmissions, with an arbitrary number of receive antennas. The Meyer Application discloses a real value modulation technique wherein the real component of a received signal is separated from an imaginary component of the received signal. The measured received signal is phase shifted from the transmit signal due to channel oscillation and other factors. The received signal is then projected back onto the real axis. The methodology assumes signal and part of interference are orthogonal and, as such, the real and imaginary part of the signal can be exploited to cancel the interference. The methodology described in the Meyer application is sensitive to the actual data comprising the signal, including some of the embedded data signals such as the training sequence. Also, the complexity of the calculations appears to be significantly higher than the complexity of the present invention. Because of this, the technique is unreasonable to implement in a large-scale wireless telecommunication system.
Another method of interference cancellation, involving at least two antenna systems, is combining algorithms, including a “switching combining algorithm” wherein one of the signals is ignored at any given point in time, and an “interference ratio combining algorithm wherein each signal is whitened in accordance with its signal-to-interference ratio. Again, the problem with these types of combining algorithms is that they require the observations to be independent in order to provide reasonable gain. As will be appreciated by those skilled in the art, the ability to receive highly independent receive signals within the small footprint of a handset is a very challenging task.